Wastewater treatment — dissolved oxygen (DO) in sewage: select the correct comprehensive statement

Introduction / Context:
Dissolved oxygen (DO) in wastewater is a critical indicator for biological processes and receiving water quality. Adequate DO in treated effluent prevents septic conditions, odours, and fish kills when discharged into surface waters, and it reflects effective secondary treatment and aeration control.

Given Data / Assumptions:

• “ppm” is used synonymously with mg/L in dilute aqueous systems.
• Fresh domestic sewage rapidly depletes DO via biochemical oxygen demand (BOD).
• Impurities reduce oxygen solubility relative to pure water.

Concept / Approach:

Properly treated secondary effluent typically targets a DO of about 4–5 mg/L leaving the plant to avoid oxygen sag in the receiving stream. Raw sewage, unless extremely fresh, usually shows negligible DO because microbes consume oxygen quickly. Solubility of oxygen in saline/impure water is modestly lower than in distilled water; “about 95%” is a reasonable instructional approximation acknowledging reduced solubility due to dissolved constituents.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Standard design manuals (activated sludge) recommend maintaining DO of 1.5–3 mg/L in aeration tanks and ≥4 mg/L in final effluent to prevent septicity downstream.

Why Other Options Are Wrong:

Choosing one statement alone ignores the broader context; “None of these” contradicts established practice and physical chemistry.

Common Pitfalls (misconceptions, mistakes):

Assuming raw sewage can sustain high DO; overlooking the effect of temperature and salinity on DO solubility.

Final Answer:

All the above